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I thought that's what you were referring to Joel! When I was a lot younger, I took a break from welding for about seven months and I learned how to inspect, calibrate and resurface only granite surface plates up and down the East coast, and all the way west to Chicago... We used a cast Iron plate that had a slightly concave surface shape... We would then impregnate first 10 micron then 35 micron diamond powder onto the surface of the cast Iron plate with a steel roller, but I'm getting ahead of myself because, before we had to do any sort of cutting on the granite surface, we first had to inspect the plate according to the grade it was classified as by NIST.
Now, we would use a laser auto-collimator with a set of mirrors, or just one for smaller sized plates in order to plot out the surface of the granite by checking the 8 axis of the rectangular surface... In other words, the 4 perimeter dimensions, then the two length and width dimensions and finally the two diagonal dimensions... By plotting every one to four inches depending on the size of the plate on to a sheet of graph paper, we would then have an overall surface shape of the granite surface plate!
The pink and The blue-gray new hampshire or vermont granite are fo the best quality because they hold their surface shape batter than any other typ ofe granite, and the Black Gabbro bing the worst garde of granite because it loses it's surface shape in a relatively short period of time... Sometimes in less than a year depending on how much mechanical inspection is going on it throughout the year... Now the classifications go like this laboratory, inspection and tool grades and each of these grades have to abide to a specific overall surface flatness tolerance for it's particular size with the lab grade having the tightest tolerance to uphold and maintain
Once the poltting of the surface is complete, one can easilysee on the graph paper where the peaks and valleys or grooves are located on the surface of each plate... This helps in determining the best possible way to resurface the plate so that the tolerance for that specific plate can be restored by resurfacing via the use of the diamond impregnated cast iron cutting plates which come in a variety of sizes depending on how much surface one must cut... Usually the surface contour is restored to a slightly convex shape and the majority of plate that require surfacing are the lower grade Black Gabbro Granite plates which IMHO, shouldn't even be relied upon for more than a six month period because they wear down and go out of tolerance easily before a year has gone by!!! Like I said before, the Pink and blue-gray granite from New Hampshire and Vermont are the best grades by far because they take years before they go out of tolerance and require resurfacing!!!
My favorite job was resurfacing plates which were used to inspect USN submarine periscopes near Charlottesville, VA when Sperry used to make them because they were pink New Hampshire granite in a temperature controlled environment as well... In other words, in a clean room which had to be shut down if the plates required resurfacing and that one visit I had required on of them to be resurfaced, so for an entire week, they had to shut down their inspection clean room where I had to work and the reason being was that when the granite is being cut by these diamond impregnated cast iron plates, the amount of dust produced is tremendous, and would have clearly wreaked havoc on their filtration system, so that's why I had the entire room all to myself!!!:) So that's what working on Granite surface plates is about in a nutshell... There's more to it but, I condensed it just so you could get a better feel for it. ;)
If you think some welding fums are so toxic, then you haven't resurfaced granite surface plates or breathed in any granite dust combined with diamond powder which was far worse, and back in those days PPE's were considered a luxury to most employers as well as a hassle until the Tsunami of lawsuits followed shortly afterwards! :(
It was a fun job I must say, but some of the scheduling was so friggin unrealistic as well as the horrible exposure conditions which were the primary reasons why I left that company to go bac kinto welding and I haven't looked back since! ;)
Respectfully,
Henry